Effect of Electronic Structure on the Photoinduced Ligand Exchange of Ru(II) Polypyridine Complexes

The series of complexes [Ru(bpy)₂(L)]²⁺, where bpy = 2,2′-bipyridine and L = 3,6-dithiaoctane (bete, <b>1</b>), 1,2-bis(phenylthio)ethane (bpte, <b>2</b>), ethylenediamine (en, <b>3</b>), and 1,2-dianilinoethane (dae, <b>4</b>), were synthesized, and their photochemistry was investigated. Photolysis experiments show that the bisthioether ligands in <b>1</b> and <b>2</b> are more easily photosubstituted by chloride ions, bpy, and H₂O than the corresponding diammine complexes in <b>3</b> and <b>4</b> to generate the bis-substituted products. Electronic structure calculations show that bond elongation in the lowest energy triplet metal-to-ligand charge transfer (³MLCT) state compared to the ground state is greater for complexes containing bisthioether ligands than those with coordinated bidentate nitrogen atoms. This elongation in the excited state is attributed to Ru–S π-bonding character of the highest occupied molecular orbitals, which is not present in the diamine complexes. In the Ru→bpy ³MLCT state, the lower electron density on the metal-centered highest occupied molecular orbital (HOMO) weakens the Ru–S bond and results in the greater photoreactivity of <b>1</b> and <b>2</b> relative to that of <b>3</b> and <b>4</b>. The more efficient photoinduced ligand exchange of the complexes possessing thioether ligands results in binding of <b>1</b> and <b>2</b> to DNA upon irradiation.